Thin film transistor liquid crystal Display (TFT-LCD) has become one of the most popular and modern information goods. As result of being light, small and portable, having a lower operating voltage, being free of harmful radiation and suited to production on large scale, TFT-LCD substitutes for cathode ray tube display as a caressed computer display device.
In accordance with the structure of TFT-LCD, Drain of TFT has a higher electric field while TFT is operating, and there should be an off-state leakage current resulted while the device is shut down, thereby the application of TFT-LCD being limited.
Presently, someone provides a lightly doped drain structure and a field induced drain structure for preventing TFT-LCD from the off-state leakage current. FIG. 1 illustrates a lightly doped drain structure of the prior art for solving the problem of the off-state leakage current. The structure includes an insulating substrate 11, a source/drain layer 12, a gate insulating layer 13 and a gate layer 14, wherein the source/drain layer 12 further includes a drain 121, a lightly doped drain 1211, a channel 122, a source 123 and a lightly doped source 1231. The electric field of the drain 121 is reduced by means of adding lightly doped regions (i.e. the lightly doped drain 1211 and the lightly doped source 1231) corresponding to the original source 123 and the original drain 121 respectively near the channel 122, so as to prevent from the leakage current. However the TFT-LCD with the lightly doped regions is complex and hard to manufacture. Furthermore the resistance will increases because of the lightly doped degree. As result of the series resistance of the drain 121 and the source 123 increasing, the operating speed of the device reduces and the power dissipation increases.
Moreover, another improving structure of field-induction drain has been disclosed. However it has to add an extra photolithographic process for manufacturing the improving structure. The more photolithographic processes are introduced, the more mis-alignment and infected defects are resulted. Therefore, the cost and the manufacturing time of the improving structure must increase and the yield reduces.
Hence, the present invention is attempted to improve the prior art and provides a structure of a thin film transistor applied to a TFT-LCD and a manufacturing method thereof for preventing TFT-LCD from the leakage current.